Not applicable.
Not applicable.
The field of the invention is corona and plasma treater stations, and particularly, the cooling and selective operation of such stations.
Materials, such as paper, polymer webs and various films, are often used in printing, coating and laminating applications. Many polymers, for example, have chemically inert surfaces with low surface energies providing poor bonding properties. To improve bonding, the surface of the material must be treated to raise the surface tension and thereby improve the application of adhesives and inks. There are a number of known techniques for treating the surface of materials to raise surface energy including flame, chemical, corona and plasma treatments.
In corona treatment, the material is ordinarily fed through a treatment zone in which one surface of the material is bombarded with ions produced by a high voltage alternating electric field ordinarily operating at frequencies of 10-50 kHz. The material is supported in the treatment zone by a roller, which also serves as one electrode for the treater. An active electrode assembly is supported in the treatment zone and spaced from the roller equidistantly along its length. The active electrode may be either a single integral electrode that extends the entire width of the treater station, or it may be a set of electrode segments, which can be selectively moved into and out of a treatment position to adjust the location, and width of the treatment zone as disclosed in U.S. Pat. No. 3,409,537. In any event, a high voltage ac power supply is connected across the roller and active electrodes to produce a corona through which the web passes.
Plasma treaters provide a more uniform and controllable surface treatment that corona treaters. In plasma treaters, the treatment zone is infused with an inert gas that is partially ionized by the energized electrodes. Early plasma treaters required that the pressure of the ionized gas was reduced well below atmospheric pressure. This required expensive and cumbersome vacuum chambers and pumps to maintain the low pressure at the treatment zone.
Treaters forming plasma in the treatment zone at atmospheric pressure were subsequently developed, for example, as disclosed in U.S. Pat. No. 5,456,972. The plasma would form at atmospheric pressure provided an inert gas, typically helium, was used, a dielectric was applied between the electrodes and the operating frequency and voltage of power source was properly selected. Moreover, in the system of U.S. Pat. No. 5,789,145, the gas had to be forced through properly sized and spaced openings in the electrodes.
The art of atmospheric plasma treaters was again advanced by the system disclosed in U.S. Pat. No. 6,118,218 in which a gas or gas mixture was forcibly diffused through active electrodes made of a porous metal. This patent explains that as the gas passes through the small pores of the electrodes (on the order of a micron), a hollow cathode effect is produced which facilitates ionization of the gas so that a atmospheric plasma can be produced at low frequencies.
In all of these systems, a major design objective is to produce a certain treatment level with the minimum amount of input power. As a general matter, to achieve a higher treatment level with a given treater at atmospheric pressure, more electrical power is applied. Higher power levels produce more heat and methods must be used to dispose of this heat. Such methods include cooling the active electrode with external air, cooling the active electrode with cooling fluids, and the addition of active electrodes disposed around the circumference of the roller electrode to disperse the heat. All of these measures increase the complexity, size and cost of the treater; and it can introduce contaminants into the electrode assembly. Moreover, external air cooling is not suitable for plasma treaters because the plasma could be evacuated from the treatment zone.
One aspect of the present invention is a treater having an air cooling system for one or more tubular active electrodes with inlet and outlet ports in communication with an inner cavity of each active electrode. Air can be passed through the active electrodes from the inlet to the outlet ports to carry away thermal energy during use.
Another aspect of the invention is a treater that can be operated in any one of three distinct modes. The active electrodes form a chamber into which gas from one or more supply lines can be pumped. The gas in the chamber is spread along the discharge surfaces of the active electrodes by a diffuser made of a porous ceramic. Flow through the supply lines can be controlled to set the mode of use. In particular, the supply lines can be closed off so that only air is present in the treatment zone during the treatment process. In this case, the treater provides corona treatment. Alternatively, a gas or gas mixture including helium can be piped to the treatment zone, in which case the treater can provide atmospheric plasma treatment. Finally, the treater can operate in a xe2x80x9cchemical coronaxe2x80x9d treatment mode in which the treatment zone is infused with a gas or gas mixture that is ionized but does not form a plasma.
One object of the invention is to provide a treater that can be operated in any one of three treatment modes: corona, chemical corona and atmospheric plasma. The treater of the invention can be operated in any one of the modes by simply controlling the flow of gas to the active electrodes and selecting the appropriate power parameters. Thus, the treatment level produced by the active electrode of a corona treater can be improved per unit of input energy by infusing the treatment zone with helium (or helium mixture) to create plasma at one atmosphere with considerably less power consumption.
Another object of the invention is to sufficiently cool the active electrodes without introducing contaminants into the assembly or using high volume exhaust system. This is accomplished by using hollow, impermeable active electrodes and transferring thermal energy using a low volume of compressed air circulating therethrough.
The foregoing and other advantages of the invention will appear from the following description. In that description reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. This embodiment does not represent the full scope of the invention. Thus, the claims should be looked to in order to judge the full scope of the invention.